This invention relates generally to transparent polyurethane compositions, and to coated transparencies and laminates incorporating such compositions, and more particularly to such compositions, coated transparencies, and laminates having antistatic or static dissipative properties.
Polyurethanes and other organic polymers generally are poor conductors of electricity. Consequently, these polymers cannot be used satisfactorily without modification in applications where static dissipative properties are required.
Several methods have been used in the past to modify polyurethanes so as to increase their electrical conductivity, and thereby to better dissipate a buildup of static charge. In one such method, conductive fibers or particles are incorporated into the polyurethane matrix. This method is not suitable for use with polyurethanes that are transparent, however, because the conductive filler materials render the modified polyurethane opaque.
In another method for modifying polyurethanes to increase their electrical conductivity, conductive polymers based on polyanilines are incorporated into the polyurethane matrix. Again, however, this method is not suitable for use with polyurethanes that are transparent, because the polyaniline additives form a dispersed phase that reduces the polyurethane""s transparency. In addition, polyanilines generally are ineffective at increasing the modified polyurethane""s conductivity when incorporated at a low concentration.
In yet another method for modifying polyurethanes to increase their electrical conductivity, hydrophilic additives such as amines and quaternary ammonium salts are used to increase the polyurethane""s surface conductivity. These additives function by migrating to the polyurethane""s surface, where they attract water and thereby create a conductive film. This method is not suitable for polyurethane coatings and laminates, however, because the additive also migrates to the surface of the polyurethane that interfaces with the underlying substrate, to cause a loss of adhesion. In addition, such additives can lose their effectiveness over time, because they can leach from the polyurethane under normal use conditions.
Still other methods for modifying polyurethanes to increase their electrical conductivity, usable in the past only for polyurethane foams, call for adding in ionizable metal salts coupled with an enhancer. The preferred salt cation is an alkali or alkaline earth metal ion, and the preferred anion is the conjugate base of an inorganic acid or a C2-C4 carboxylic acid. The preferred enhancers are phosphate esters and salts or esters of fatty acids.
None of these known additives for increasing the electrical conductivity of polyurethanes are considered fully satisfactory for use in polyurethanes that are transparent, and particularly in polyurethanes that are used as coatings or in laminates for aircraft windows.
In general, non-ionic additives and polyol modifiers have been found to significantly enhance electrical conductivity only if used at high levels, which can adversely affect other important properties, such as transparency and mechanical strength. Ionic additives, including quarternary ammonium salts and ionizable metal salts, generally are more effective in enhancing electrical conductivity. The most effective known additives of this kind are ionizable metal salts of perfluoroalkylsulfonates. However, none of these ionic additives are considered fully satisfactory for use in transparent polyurethanes used as coatings or in laminates, because with aging they can cause a loss of transparency and a loss of adhesion.
It should therefore be appreciated that there is a need for an improved polyurethane composition, and for coated transparencies and laminates incorporating such a composition, that incorporates an additive for enhancing electrical conductivity without adversely affecting the composition""s transparency and without adversely affecting the composition""s adhesion to an underlying substrate. The present invention fulfills this need and provides further related advantages.
The present invention is embodied in an improved polyurethane composition, and in coated transparencies and laminates (e.g., aircraft windows) incorporating such a composition, the composition incorporating a prescribed additive for enhancing electrical conductivity without adversely affecting the composition""s transparency and or adhesion to an underlying substrate. More particularly, the polyurethane composition incorporates 0.5 to 5.0 weight percent of an ionizable salt of a perfluoroalkylsulfonimide. The metal preferably is an alkali metal, and the perfluoroalkylsulfonimide preferably is trifluoromethanesulfonimide. The most preferred ionizable salt is lithium trifluoromethanesulfonimide, in a weight percent in the range of 1.0 to 3.0. This enhances the composition""s electrical conductivity by at least about two orders of magnitude.
The ionizable salt of a perfluoroalkylsulfonimide is suitable for use as an additive in both polyesterurethanes and polyetherurethanes. The preferred polyurethane composition incorporates aliphatic polyetherurethane, with which the prescribed additive can reduce electrical volume resistivity to values of less than about 1xc3x971011 ohm-cm.
One important use of the polyurethane composition of the invention is as a transparent coating or as part of a transparent laminate. When used as a transparent coating, the composition has particular utility when coated onto a transparent, conductive metal coating, e.g., indium tin oxide (ITO), gold, and gold/metal oxide stacks, overlaying a transparent substrate. The enhanced electrical conductivity minimizes the risk that static charge can build to a point where a shock hazard is created or the polyurethane coating is damaged by a rapid discharge of electrical current.
Other features and advantages of the invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.